Three-pump hydraulic system incorporating an unloader



Oct. 27, 19 70 BECKER ET AL. 3,535,877

THREE-PUMP HYDRAULIC SYSTEM INCORPORATING AN UNLOADER Filed May 9, 1969I EEEM DEMAND VALVE 23 l5 l6 I7 l8 ENGINE FIG! TNVENTORS LANSON BECKERROBERT W. RUE

ATTORNEYS United States Patent 3,535,877 THREE-PUMP HYDRAULIC SYSTEMINCORPO- RATING AN UNLOADER Lanson Becker, Galesburg, and Robert W. Rue,Kalamazoo, Mich., assignors to General Signal Corporation,

a corporation of New York Filed May 9,1969, Ser. No. 823,401 Int. Cl.F15b 13/09 U.S. C]. 6052 7 Claims ABSTRACT OF THE DISCLOSURE Anintegrated hydraulic system for a vehicle including steering andimplement circuits and three supply pumps. Two of the pumps areconnected, respectively, with the two circuits, and a demand valvesplits the output of the third pump between the circuits as needed tomaintain a substantially constant rate of flow to the steering circuit.An unloader diverts to tank either the output of the third pump or theimplement output of the demand valve when implement pressure exceeds apredetermined value, and delivers said output to the implement circuitwhen that pressure reduces to a predetermined lower value.

BACKGROUND AND SUMMARY OF THE INVENTION U.S. Pat. 3,355,994, grantedDec. 5, 1967, discloses an integrated hydraulic system for constructionvehicles, such as articulated loaders, which includes a pair of pumpsfor supplying oil exclusively to the steering and implement circuits,respectively, and a flow dividing or demand valve for splitting theoutput of a third pump between the two circuits as needed to maintain asubstantially constant rate of flow to the steering circuit. Althoughthis system aifords real advantages, its use on some articulated loadershas been accompanied by excessive heating of the hydraulic fluid. Somepeople have attributed this difficulty to the steering circuit evidentlbecause the flow rates usually are higher than in the correspondingcircuits for non-articulated vehicles, and also because metering ofthese flows commonly is accompanied by large pressure drops. However,our study of the problem compels a different conclusion and provides asound basis for obtaining relief.

Commonly, the work cycle of an articulated loader is of twenty secondsduration and is divided into the following four phases: (1) the timespent working in a pile of material filling the bucket, (2) the timerequired to move from the .pile to a truck, (3) the time spent at thetruck dumping the material, and (4) the time consumed returning to thepile. Our investigation indicates that, on a typical loader whose enginehas low and 'high idle speeds of 600 and 2200 r.p.m., respectively, theoperating conditions of the hydraulic system during a representativework cycle are as follows:

This table shows that the steering circuit consumes only about 20% ofthe total hydraulic energy expended during the work cycle, and thusdemonstrates clearly that the steering circuit is not the cause of theheating problem. Just as importantly, the table indicates that the majorportion of the implement energy is spent while the loader is at the pileand the bucket is being filled. Since the bucket manipulations requiredfor the filling operation require maximum pressure, but only very smallrates of flow, it follows that most of the large volume of oil deliveredto the implement circuit in this phase of the cycle is diverted to tankacross the circuits relief valve. This, then, is the real cause of theoverheating problem.

The object of this invention is to provide an improved version of thethree-pump system of Pat. 3,355,994 which reduces the amount of energyconverted into heat during the In Pile phase of the work cycle.According to the invention, the system is provided with an unloaderwhich, inresponse to attainment of a predetermined load pressure in theimplement circuit, diverts to tank at a low pressure drop either theoutput of the implement pump or the implement output of the demandvalve. The choice between these alternatives depends upon which outputis larger. The pressure setting of the unloader is below the setting ofthe implement relief valve and above the pressure required to lift aloaded bucket. Thus, during the In Pile phase, the volume of oildiverted to tank across the relief valve will be reduced considerably,and energy will be conserved. On the other hand, during thePile-To-Truck phase, in which the loaded bucket is raised, the implementcircuit will be supplied at the usual maximum rate so that thebucket-raising operation is not prolonged.

BRIEF DESCRIPTION OF THE DRAWINGS The preferred embodiment of theinvention is described herein with reference to the accompanying drawingin which:

FIG. 1 is a schematic diagram of an improved system in which theunloader is interposed in the implement output conduit of the demandvalve.

FIG. 2 is a simplified schematic diagram of an improved system in whichthe unloader is interposed in the output conduit of the implement pump.

DESCRIPTION OF PREFERRED EMBODIMENT The improved system shown in FIG. 1is arranged to supply fluid to the open center directional controlvalves 11 and 12 of the steering and implement circuits 13 and 14,respectively. Although the illustrated implement circuit 14 has only asingle valving unit 12, it will be understood that the actual circuitincludes a valving unit for each implement function. The system includesthree fixed displacement supply pumps 1517, normally of the gear type,which are driven by the vehicles propulsion Implement Steering Eng.speed, Time, Pressure, Supply, Pressure, Supply, Work cycle phase r.p.m.sec. p.s.1. g.p.m. p.s.i. g.p.m

In pile 1, 700 5 2,000 72 100 26 Pile-to-truek 2, 200 2 1, 000 94 100 332, 000 2 100 2, 000 30 2,100 2 100 32 At truck 700 1 500 16 500 25 1,200 1 500 44 500 26 2, 200 1 500 94 500 33 'lruek-to-pile 2, 200 2 10094 100 33 2, 100 2 100 90 500 32 2, 100 2 100 90 100 32 3 engine 18 andwhich draw fluid from the common reservoir tank 19 to which the valves11 and 12 exhaust. The output of pump 15 is delivered continuously tocontrol valve 11 through conduits 21 and 22 and the demand valve 23, andthe output of pump 17 is delivered continously to implement valve 12through conduit 24. Pump 16, on the other hand, is connected with demandvalve 23 through conduit 25, and, depending upon the speed of engine 18,its output is delivered to either or both of the valves 11 and 12through conduit 22 or conduit 26. The demand valve 23 can take any oneof various known forms, but, in any case, it serves to maintain asubstantially constant total rate of fiow to valve 11 throughout a majorportion of the speed range of engine 18. For example, if the low andhigh idle speeds of engine 18 are 600 and 2200 r.p.m., respectively,valve 23 may be designed to maintain a constant steering supply rate atspeeds between 700 and 1700 rpm. At higher speeds, switch pump 16 isisolated from the steering circuit by valve 23, and the total output ofthe pump is delivered to conduit 26.

In the FIG. 1 system, switch pump 16 is larger than implement pump 17,so the unloader 27 provided by the invention is interposed in theconduit 26 leading from demand valve 23 to implement circuit 14.Although various types of unloaders can be used, the device 27 chosenfor illustration comprises a through passage 29 leading from conduit 26to implement circuit 14, an unloading valve 31 which is adapted to openand close an exhaust passage 32 interconnecting passage 29 and tank 19,and a check valve 33 which is interposed in passage 29 and is orientedto prevent escape of oil from circuit 14 at times when valve 31 is inunloading position. Unloading valve 31 is biased to the illustratedloading position by a spring 34 and is actuated by a pair of opposedpressure motors 35 and 36. Motor 35, which urges valve 31 to unloadingposition, is connected with passage 29 and responds continuously to thepressure upstream of check valve 33, whereas motor 36 is subjected topressures developed by a piloting circuit which responds to the loadpressure in implement circuit 14. The piloting circuit includes a pilotpassage 37 which leads to motor 36 from passage 29 and contains a flowrestrictor 38, and a second pilot passage 39 which leads from motor 36to tank 19 and is controlled by pilot valve 41. The pilot valve isbiased closed by a spring 42 and is opened by the pressure of the fluidwhich is transmitted to it through pilot passage 39 and which acts uponits nose 41a. Since this pressure is essentially the same as the loadpressure in implement circuit 14, it will be evident that pilot valve 41opens when implement pressure reaches a predetermined level. And, sincethe pilot passage 37 is restricted, it also should be evident thatopening of valve 41 will unbalance the pressures to which motors 35 and36 are subjected and enable motor 35 to shift valve 31 to unloadingposition. After valve 41 has been opened, it

is held open by a piston 43 which is subjected to the load pressure inimplement circuit 14 through a passage 44. The cross sectional area ofpiston 43 is larger than the effective area of valve nose 41a, so thepressure at which valve 41 closes will be materially lower than thepressure at which the valve opens. This differential is necessary inorder to make unloader 27 stable.

In order for unloader 27 to be effective as an energy conservator, valve31 must shift to unloading position at a load pressure below the settingof the implement relief valve 45. On the other hand, since rapid raisingof the loader bucket requires the outputs of both of the pumps 16 and17, valve 31 must shift to loading position at a pressure above thatneeded to lift the bucket. In a typical case wherein relief valve 45 isset for 2,000 p.s.i. and the maximum pressure required for liftingoperations is 1,000 p.s.i., spring 42 and the differential between theeffective areas of piston 43 and valve nose 410 are so selected thatpilot valve 41 opens at a pressure of 1,800 p.s.i. and closes at apressure of 1,500 p.s.i.

When the FIG. 1 system is in service and the loader commences working ina pile of material, the load pressure in implement circuit 14 will riseabove the 1,800 p.s.i. setting of unloader 27, and pilot valve 41 willopen. As mentioned earlier, this action reduces the pressure at motor 36relatively to the pressure at motor 35 and enables the latter to shiftvalve 31 to unloading position. Therefore, the implement output ofdemand valve 23 will be diverted to tank 19 through passage 32 at arelatively low pressure drop. Since, during this bucket-fillingoperation, the speed of engine 18 usually is relatively high, most ifnot all of the output of switch pump 16 will follow this unloading path.This reduces drastically the volume of oil which escapes to tank 19through relief valve 45, and consequently conserves energy.

When the bucket-filling operation has been completed, the load pressurein implement circuit 14 will decrease below 1,500 p.s.i., and spring 42will overpower piston 43 and close pilot valve 41. Closure of pilotpassage 39 effects equalization of the pressures at motors 35 and 36, sospring 34 now shifts valve 31 back to the illustrated loading position.As a result, the implement output of demand valve 23 now passes toimplement circuit 14 via conduit 26, passage 29 and check valve 33.Therefore, as the loader withdraws from the pile, the operator may raisethe bucket at a rapid speed utilizing the full outputs of both of thepumps 16 and 17. And, in the usual case, unloader 27 will remain in thisloading condition throughout the remainder of the work cycle.

Although, in most installations switch pump 16 is larger than implementpump 17, sometimes the relative capacities of the pumps are reversed. Inthese cases, the implement output conduit 26 of demand valve 23 isconnected directly to the implement circuit, and unloader 27 isinterposed in the output conduit 24 of implement pump 17. Thisarrangement is illustrated in FIG. 2.

The unloader 27 can be mounted at various places on the vehicle, but thepreferred location is at the implement valve 12. This arrangementpermits the two valves to share a common tank line, and allows theunloader to serve either of the pumps 16 and 17 merely by interchangingconduit connections.

We claim:

1. In an integrated hydraulic system of the type includmg first andsecond work circuits (13, 14) connected to receive oil from first andsecond pumps (15, 17), respect vely, and a demand valve (23) connectedwith the two circuits and adapted to split the output of a third pump(16) between them as required to maintain a substantially constant totalrate of flow to the first circuit (13), the improvement which comprisesan unloader (27) interposed either in the connection (26) between thedemand valve (23) and the second work circuit (14) or in the connection(24) between the third pump (17) and the second Work circuit, theunloader being responsive to the load pressure in the second workcircuit (14) and serving to divert to a reservoir (19) the oil deliveredto the associated connection (24 or 26) when the pressure rises above afirst value and to deliver said oil to the second work circuit when thepressure reduces below a second, lower value.

2. The improvement defined in claim 1 in which:

(a) the third pump (17 is in continuous communication with the secondwork circuit (14); and

(b) the unloader (27 is interposed in the connection (26) between thedemand valve (23) and the second work circuit.

3. The improvement defined in claim 1 in which:

(a) the connection (26) between the demand valve (23) and the secondcircuit (14) is continuously open; and

(b) the unloader (23) is interposed in the connection (24) between thethird pump (17) and the second work circuit (14).

4. The improvement defined in claim 1 in which the unloader (27comprises:

(a) an unloading valve (31) for opening and closing an exhaust path (32)leading from the associated connection (24 or 26) to the reservoir (19);

(b) means including a first pressure motor (36) urging the unloadingvalve closed;

() a second pressure motor (35) subject to the pressure in theassociated connection-(24 or 26) and urging the unloading valve open;and

(d) pilot valve means (37-39, 41) for subjecting the first pressuremotor (36) to the pressure in the associated connection (26) when thepressure in the second circuit (14) decreases below said second value,and for subjecting the first pressure motor (36) to a lower pressurewhen the pressure in the second circuit increases above said firstvalue.

5. The improvement defined in claim 4 in which the (b) means (41a)responsive to the pressure in said second portion (39) for opening thepilot valve (41) when the pressure rises above said first value; and

(c) means (43) responsive to the pressure in the second circuit (14) forholding the pilot valve (41) open until the pressure reduces to saidsecond value.

7. The improvement defined in claim 6 in which:

(a) the unloader (27) includes a check valve (33) interposed in theassociated connection (24 or 26) between the junction with said exhaustpath (32) and the second circuit (14) and oriented to prevent flow fromthe second circuit; and

(b) the pilot passage (37, 39) joins the associated connection at theupstream side of the check valve (33).

References Cited UNITED STATES PATENTS unloader (27) also includes acheck valve (33) interposed in the associated connection (24 or 26)between the junction with said exhaust path (32) and the second circuit(14) and oriented to prevent flow from the second circuit.

6. The improvement defined in claim 4 in which the pilot valve meanscomprises:

(a) a pilot passage including a first portion (37) leading from theassociated connection (24 or 26) to the first pressure motor (36)through a flow restriction (38), and a second portion (39) leading fromsaid pressure motor (36) to the reservoir (19) and being normally closedby a pilot valve (41);

EDGAR W. GEOGHEGAN, Primary Examiner US. Cl. X.R.

